One way to establish the capacitance of what is commonly referred to as a parallel plate capacitor is by changing the spacing between the capacitor plates or electrodes. There are capacitors of this type that may be tuned (to vary the capacitance) by changing the spacing between the capacitor electrodes. Known tunable configurations utilize a pair of capacitor electrodes, at least one of which is movable relative to the other to change the distance therebetween, and to thereby change the capacitance. Suspension springs may be used to movably interconnect any such capacitor electrode with a substrate to allow the particular capacitor electrode to be moved relative to the other capacitor electrode. In cases where the electrical signal to a capacitor electrode is directed through such a suspension spring, the quality of the capacitor is typically degraded or reduced because of the increased series resistance caused by the configuration of suspension spring.
Another known effect on the capacitance of a parallel plate capacitor is what, if anything, is disposed between the pair of capacitor electrodes. Changing what is disposed between the capacitor electrodes changes the capacitance of the capacitor. It is also known to have a pair of capacitor electrodes that are spaced in the vertical dimension, and to vary the capacitance by moving some type of structure in the horizontal dimension into the space between the vertically spaced capacitor electrodes. The amount that this structure is advanced within the space between the vertically spaced capacitor electrodes also has an effect on the capacitance of the tunable, parallel plate capacitor.
There are many applications that would benefit from the ability to fabricate tunable capacitors of a relatively small size. A number of microfabrication technologies have been explored for fabricating small structures in general (e.g., “microstructures”, such as micromechanical devices or microelectromechanical devices) by what may be characterized as micromachining. Representative micromachining techniques include without limitation LIGA (Lithographie, Galvonoformung, Abformung), SLIGA (sacrificial LIGA), bulk micromachining, surface micromachining, micro electrodischarge machining (EDM), laser micromachining, and 3-D stereolithography. Bulk micromachining has been utilized for making relatively simple micromechanical structures. Bulk micromachining generally entails cutting or machining a bulk substrate using an appropriate etchant (e.g., using liquid crystal-plane selective etchants; using deep reactive ion etching techniques).
One micromachining technique that allows for the formation of significantly more complex microstructures is surface micromachining. Surface micromachining generally entails depositing alternate layers of structural material and sacrificial material using an appropriate substrate which functions as the foundation for the resulting microstructure. Various patterning operations may be executed on one or more of these layers before the next layer is deposited so as to define the desired microstructure. After the microstructure has been defined in this general manner, the various sacrificial layers are removed by exposing the microstructure and the various sacrificial layers to an appropriate etchant (e.g., one that is biased to the sacrificial layer(s)). This is commonly called “releasing” the microstructure from the substrate, typically to allow at least some degree of relative movement between the microstructure and the substrate.